Spectral sensitivity characteristics of an image pickup device having sensitivity to not only the visible region but also the infrared region largely depart from the color match sensitivity. For this reason, in order to take a color image close to what human eyes would see using such an image pickup device, an infrared cut filter is generally disposed in the optical path extending to the image pickup device. In addition, by providing a mechanism to move the infrared cut filter out of the optical path, it is possible to use the image pickup device for both color imaging and highly sensitive imaging, such as night vision imaging.
On the other hand, another method to acquire a color image using the above-mentioned image pickup device is to apply color correction by signal processing to image signals obtained by the image pickup device. Compared to disposing an infrared cut filter, the color correction by signal processing has an advantage in allowing the size of an image pickup apparatus to be reduced, and, in general, has an advantage also in terms of the manufacturing cost. Further, the color correction method facilitates switching between color imaging and highly sensitive imaging.
One example of the color correction method by signal processing is to perform signal processing using matrix coefficients. For example, a technology has been proposed in which matrix coefficients are calculated by the method of least squares based on a relationship between colors in an image obtained by imaging without the use of an infrared cut filter (correction object colors) and colors in an image obtained by imaging with the use of an infrared cut filter (target colors) and the color correction is performed using the calculated matrix coefficients.
Please see, for example, yoshitaka Toyoda et al., “Near infrared cameras to capture full color images—A study of color reproduction methods without an infrared cut filter for digital color cameras”, The journal of the Institute of Image Information and Television Engineers, pages 101-110, 2010-01, The Institute of Image Information and Television Engineers.